The invention relates generally to the area of programmable machine function controllers; and specifically, the invention provides an improvement thereto for detecting signal failures.
Diagnostic apparatus for sequential and cyclic operations have been known for many years. Typically, each cycle of operation is divided into a number of sequences or cycle steps, and a visual display is provided upon the successful completion of each sequence. Therefore, if the cycle stops, by observing the visual display, the operator can begin to diagnose the problem. However, the systems are unable to determine which signal in the system is missing and, therefore, preventing the cycle from progressing.
Through the years, these controls have developed in various ways. Typical of the development is the apparatus illustrated in U.S. Pat. No. 3,719,931 which illustrates a sequential machine function controller. Upon application of power, the control increments to a first sequence. This causes one or more output signals to be generated. Further, other signal lines representing desired input signals are energized in their proper order. Associated with the signal lines is a first set of visual indicators. Input signal converter circuits are responsive to the machine devices for producing the actual input signals. A second set of visual indicators is responsive to the converter outputs. A coincidence circuit compares the desired input signals with the occurrence of the actual input signals; and if complete coincidence exists, the control is incremented to the next sequence which generates a new set of output signals in a corresponding new set of desired input signals. If the control stops, the operator compares the two sets of visual indicators; and using a legend identifying the signals associated with the visual indicators, he then initiates further diagnostic action.
U.S. Pat. No. 3,939,453 discloses a similar system with several changes thereto. First, the multiple visual indicators are replaced by a single numeric display to identify the desired input signal which has not been satisfied by a corresponding actual input signal. Second, the display is further operative to numerically identify the current sequence of the control.
The systems described in these patents have several limitations. First, because the control is embodied by fixed circuit elements, its progression through the cycle of sequences is controlled by the rate at which the desired input signals are satisfied. This means that only a single error condition can be displayed at one time. Further, the systems can only control and diagnose a single independent cycle of operation. Independent and simultaneously occuring cycles of operation require a duplication of the elements illustrated. Finally, the prior art does not illustrate how the diagnostic system may be used to diagnose failures which are independent of any particular cycle of operation.
With the more recent commercial exploitation of programmable machine function controllers, a diagnostic system is feasible which has none of the limitations of the prior art devices. Typically, machine function controllers have a memory containing a machine function program which is continously scanned by the controller. The program defines which combination of input signals are required to generate an output signal. At points in time, when the programmed combination of input signals simultaneously exist, the controller generates the corresponding programmed output signal.
Using the diagnostic apparatus disclosed herein, an improved programmable machine function controller is provided which detects failures in independent but simultaneously occurring cycles of operation associated with the machine. Alternatively, if the controller is connected to a plurality of simultaneously operating machines, the diagnostic apparatus will continously display error conditions associated with the operation of all the machines. In addition, the diagnostic program gives the versatility of testing only certain inputs or only certain cycles of operation which may be chosen as a function of the known reliability of the elements involved. Further, because the diagnostics are defined by a separate diagnostic program in the controller, the diagnostic program may be easily changed to conform to changes in the machine control program. The diagnostic system operates asynchronously with the controller; therefore there is no real time correlation between the diagnostics operation and the machine control functions. This simplifies the machine control design problem and permits the machine control and diagnostics program to be designed separately. Finally, the asynchronous operation of the diagnostic apparatus permits the same apparatus to check failures of input signals which are not related to particular cycles of operation.